MAU and DAU or "monthly average users" and "daily average users" are the two canonical metrics that evaluate the success or failure of a Facebook application. Both of these numbers are reported by Facebook and measure the number of users who are interacting with a given Facebook application in the specified time period. Here's an interesting 3rd party analysis of them.

For both MAU and DAU, an increasing value is important but there's an issue with that -- on January 17, 2013, Facebook stopped reporting specific MAU and DAU values and moved to "banded MAU and DAU" values. What this means is that Facebook groups MAU and DAU values into bands (10 million +, 5 million +, 1 million + and so on). This means that a top tier application like FarmVille 2 is reported as the exact same MAU value as a lower MAU value application liked Scribd. Here's an example of viewing banded applications on our Facebook Apps leaderboard.
To help better understand MAU and DAU, AppData now offers accurate MAU and DAU Estimates.